1. Field of the Invention
The present invention relates to a semiconductor structure design, and more particularly relates to a semiconductor structure avoiding poly stringer formation in the mask read only memory process.
2. Description of the Prior Art
Semiconductor devices typically include multiple individual components formed on or within a substrate. Such devices often comprise a high density section and a low density section. For example, a memory device comprises one or more high density core regions and a low density peripheral portion on a single substrate. The memory cell array is positioned in the high density core region.
As the size of the semiconductor structure shrinks, the density of the high density section is relatively raised. However, many problems about the design of the density of the device arise in the process. For example, during the memory cell formation, the etching process of the floating gate of poly I layer is performed to remove the redundant portion. In this moment, since the distance between memory cells is shrunk as the density is raised, poly stringer formation will occur during the etching process. The poly strings are formed due to incomplete removal of the poly I layer of the unmasked portions during the etching process. This phenomenon occurs during the self-aligned etch (SAE) step.
Referring to FIG. 1, a conventional semiconductor device is shown. The semiconductor device is divided into the memory cell area 10 and the peripheral portion 12 by a dotted line A–A′. In the memory cell area 10, the semiconductor substrate 14 comprises a doped area 16 and a local oxidation of silicon (LOCOS) 18 thereon. The oxide layer of the isolation device 18 comprises a gate oxide layer (not shown), a poly I layer 20, an oxide-nitride-oxide (ONO) layer (not shown), and a poly II layer 22 in sequence. In the peripheral portion 12, the semiconductor substrate 14 comprises a shallow trench isolation (STI) structure 24. When the surface of the STI structure 24 is higher than the semiconductor substrate 14, poly stringer is formed during the etching process, causing an electrically short of the memory cell. Therefore, for the high-density semiconductor structure, a solution is needed to avoid or reduce the poly stringer formation.